1. Field of the Invention
The invention relates to a temperature detector for a contact thermometer of process measurement technology having a thermowell, at least one thermal sensor element arranged in the process-side end of the thermowell and at least one electrical connecting means, wherein the electrical connecting means is connected to the thermal sensor element with a first connection side and extends in the thermowell at least up to the evaluation-side end of the thermowell, so that the thermal sensor element can be electrically contacted via a second connection side of the electrical connecting means.
2. Description of Related Art
Temperature detectors of the above mentioned type have been known for a long time in practice, wherein temperature detectors of this type come directly or indirectly into contact with the medium of which the temperature is to be determined. Contact thermometers should be presently differentiated from such thermometers that detect only thermal radiation coming from a medium. The thermowell of the temperature detector surrounds the thermal sensor element, wherein the thermal sensor element is normally arranged near a closed end of the thermowell. The thermal sensor element is often affixed in the thermowell with a heat-conducting medium, such as, for example, with a heat-conducting paste or a heat-conducting adhesive. The thermowell is thus closed on the process-side end and protects the thermal sensor element from the process. Often, a temperature detector of this type, however, is also used as an insert in a further protective casing; wherein the protective casing extends into the process and permanently protects the surroundings from the process. The temperature detector is then inserted with its thermowell into the additional protective casing. The other, evaluation-side end of the thermowell usually ends in a housing, anyway, wherein an evaluation or further processing of the signal occurs.
The thermal sensor element is normally a temperature-dependent resistor or a thermocouple that can be electrically contacted from outside of the thermowell via the electrical connecting means. A compact, modular construction around a contact head is used as housing, in which an electrical connection is produced indirectly via the electrical connecting means between the outside world and the sensor element. The contact head can, however, also have a transmitter, which has the task of filtering and linearizing a signal coming from the sensor element into a standardized signal and to provide higher-ranking process monitoring.
The electrical connection means normally implements at least two electrically conductive paths between the thermal sensor element and the connections located outside of the thermowell. In implementing the thermal sensor element as a temperature-dependent resistor, a voltage or a current is applied to the thermal resistor and correspondingly, the resulting current or the resulting voltage is determined and the temperature can then be determined using the resulting resistance values. In the simplest implementation of the electrical connecting means with two electrically conducting paths, changing resistances cannot be compensated, which is why implementations are often preferred in which the electrical connecting means have three or even four electrical leads as reference resistors.
Temperature detectors known from the prior art are relatively expensive, which, on the one hand, is due to the relatively complex production, and on the other hand, however, due to the materials being used. Temperature detectors known from the prior art often have mineral insulation of the electrical connecting means in the thermowell. Here, the electrical connecting means are located, for example, in a mineral or ceramic powder and the temperature detector has to be subject to heat treatment after mounting the thermal sensor element so that the mineral or ceramic powder is drought. Apart from the electrical connecting means with mineral insulation being expensive, they are also difficult to deal with in production, since it is necessary to make sure that the mineral powder remains in contact when mounting the thermal sensor element, which is difficult for thermowell diameters having only few millimeters to fractions of a millimeter and is practically not applicable for automation.